1. Field of the Invention
The present invention relates to the control of piezo-electric motors.
2. Discussion of the Related Art
Piezoelectric motors such as motors of type piezolegs sold by Piezomotor Company, are widely used to ensure small displacements of elements of small dimensions. Thus, such motors are used, for example, to ensure the enlargement/reduction function (zoom) of a lens of a device of small dimensions for taking fixed or animated pictures. Such devices are especially incorporated in battery-supplied portable devices having a main function other than taking pictures such as telephones or personal organizers.
FIG. 1 schematically and partially illustrates the operating principle of such a motor 1. An axis AXIS to be displaced in a horizontal direction rests on four pads P1, P2, P3, and P4. Each pad Pi, where i is equal to 1, 2, 3, or 4, rests on a piezoelectric foot Fi. The feet are arranged in two intercalated pairs F1, F3, and F2, F4. Pair F1-F3 is controlled by a voltage signal V13. Pair F2-F4 is controlled by a voltage signal V24. Signals V13 and V24 conventionally are sinusoidal signals of a 6-V amplitude and of a frequency on the order of from 80 to 100 kHz. Signals V13 and V24 typically have the same sinusoidal shapes, frequencies and amplitudes, but are phase-shifted, in phase quadrature.
A known method for controlling such a motor comprises the generation of sinusoidal signals by means of an amplifier directly connected to the motor input. Such a method consumes too high a current for mobile applications.
Rather than generating sinusoidal signals directly provided to the motor, it has been provided to obtain said signals by filtering a digital signal by means of a resonant LC filter having its capacitive elements formed by the very motor.
Such a method is described hereafter in relation with FIGS. 2, 3A, and 3B.
FIG. 2 schematically and partially illustrates an equivalent dynamic electric diagram of piezoelectric motor 1 of FIG. 1 associated with a known control circuit. Pairs F1, F3 and F2, F4 are modeled by two distinct parallel connections 2 and 3, between the same high and low supply rails Vp and GND, of a series association of two respective capacitors C1, C3 and C2, C4 and of a series association of two resistors R1, R3 and R2, R4, respectively. Capacitors C1, C2, C3, and C4 represent the equivalent capacitances of motor 1 while resistors R1, R2, R3, and R4 model the losses. Each connection 2, 3 is associated with a pulse generator (a digital state machine) respectively 4 (signal S1) or 14 (signal S2).
Generator 4 generates a periodic square signal S1 illustrated in FIG. 3A. The duty cycle of square wave S1 of FIG. 3A is equal to ½, that is, signal S1 is at a high level for a same time period (half-period) as at a low level. The output of generator 4 is connected to a first terminal of an inductive element, for example, a coil 6, having a second terminal connected to a common midpoint A1 of series associations C1-C3 and R1-R3 of connection 2. Midpoint A1 corresponds to terminals of feet F1 and F3. As a result, signal S1 is filtered by the LC filter formed of element 6 and of equivalent capacitors C1 and C3. The filtering is performed to obtain, at point A1, a perfect sinusoidal control voltage signal VA1 illustrated in FIG. 3B, of a 6-V amplitude.
Similarly, an inductive element, for example, a coil 16, is interposed between generator 14 and a common midpoint A2 of series associations C2-C4 and R2-R4 of connection 3. Midpoint A2 corresponds to terminals of feet F2 and F4. Digital signal S2 generated by generator 14 has the same duty cycle as signal S1, but phase-shifted with respect thereto to provide pair F2, F4 with a sinusoidal control voltage signal VA2 at point A2, similar to signal VA1 but in phase quadrature with respect to the latter.
To obtain sinusoidal control signals VA1 and VA2 of a given nominal frequency ranging between 80 and 100 kHz, resonant LC filters at the nominal frequency are used. Equivalent capacitances C1, C2, C3, and C4 of motor 1 being very low, on the order of 50 nF, this results in using elements 6 and 16 having a very high inductance, on the order of 30 μH.
A disadvantage of such a method lies in the fact that such elements 6 and 16 are very bulky, which is particularly disturbing in applications of optical units integrated in battery-supplied portable devices.